Wonder wire puts up no resistance

May 3, 2001

Electrical resistance arises because charge carriers - electrons and holes - collide with imperfections in the material they are travelling through. In theory then, a perfectly crystalline conductor should offer no resistance - yet for over a decade experiments have shown that virtually defect-free wires have resistances of several kilo-ohms. Now Rafael de Picciotto of Bell Labs in the US and colleagues have shown for the first time that resistance does vanish in a small but perfectly formed wire. They confirmed that the resistance detected in earlier experiments was entirely due to the resistance of the contacts of the measuring device (R de Picciotto et al 2001 Nature411 51).

Resistance-free current flow would have enormous benefits for the electronics industry and is usually associated with superconductivity. But 'ballistic quantum wires' may offer an alternative. These tiny structures are almost completely free of the defects - such as impurities and dislocations - that lead to resistance in conventional conductors. de Picciotto's team has succeeded in separating the intrinsic resistance of the wire from the resistance of the contacts through which the resistance is measured.

de Picciotto and co-workers used a special technique known as cleaved edge overgrowth to grow a layer of gallium arsenide on a sliver of aluminium gallium arsenide. This results in a layered structure with an extremely smooth flat edge. Next the team deposited tiny metal source and drain electrodes on top of the structure and applied a voltage across them. This potential difference isolates the smooth edge of the device and makes it behave as a 'one-dimensional' wire. The wire is termed 'ballistic' because the electrons can travel its whole length before meeting a defect.

Two more metal contacts are placed between the source and drain electrodes so that the resistance of the wire can be measured. When a negative voltage is applied to these electrodes, electrons are depleted from the semiconductor structure beneath, but this does not disturb the quantum wire. These depleted regions then act as resistance-free voltage probes, and this allowed de Picciotto and colleagues to measure the resistance of the quantum wire. The team's conclusion that the resistance found in previous experiments arose purely from the contacts gives valuable insight into the limits of future circuits based on quantum wires.